Ten patients who had their blood pressure controlled with enalapril (10 mg, n = 4; 20 mg, n = 6) and 10 control subjects on perindopril (4 mg, n = 6; 8 mg, n = 4) entered a double-blind crossover study. They received placebo or the active drug 1 week apart and had their blood pressure measured at 0, 2, 3, 4, and 24 h. Then they crossed over to the other angiotensin-converting enzyme inhibitors (4 mg perindopril congruent to 10 mg enalapril) and the double-blind study was repeated. Blood pressure control 2 to 4 h after drug administration was similar with both drugs (perindopril = 150 +/- 2/80 +/- 1; enalapril = 150 +/- 2/81 +/- 1). Twenty-four h after administration of perindopril blood pressure was lower than on enalapril (perindopril = 154 +/- 3/85 +/- 2; enalapril = 159 +/- 3/89 +/- 2). Enalapril, 2 to 4 h after administration, caused a greater decrease than placebo, whereas the change with perindopril did not differ from placebo. Twenty-four hours after receiving the active drug the blood pressure of subjects on perindopril did not differ from the peak effect when corrected for placebo and circadian variation, whereas the blood pressure on enalapril was higher. This study indicates that perindopril in the doses used has a longer duration of action than enalapril and is more suited to once daily use.